1. Field of the Invention
The present invention relates to a steel having good wear resistance, and more particularly to a steel and wrought steel having good wear resistance suitable for use as a material for piston rings and rocker arms of internal combustion engines, and pinion shafts for differential gears, all for parts of automobiles, as well as good fitting strength and fatigue strength.
2. Description of the Prior Art
Piston rings used for internal combustion engines consist of compression rings for maintaining the gas-tightness of combustion chambers and oil scraper rings for conditioning lubricating oil films on the wall surfaces of cylinders or cylinder liners of the engines. Of the piston rings, the compression rings are loosely fitted directly below a piston head and heavily affected by a combustion gas. Therefore, the compression rings are required to be resistant to wear (abrasive wear under the action of carbon soot and corrosive wear under the action of corrosive combustion products), scuffing, heat, etc.
With the recent trend toward internal combustion engines with lighter weight, higher outputs and higher rotating speed, development of piston rings with smaller width has been positively made. The reduction in the width of the piston ring makes it possible to reduce the weight of the piston ring, stabilize the behavior of the piston ring in a piston ring groove and to decrease the thickness of the oil film, thereby improving the lubricating oil consumption.
However, the development of the piston rings with smaller width is accompanied by a reduction in the oil film thickness, an increase of wear of the rings and a shortening of the service life of the rings. Therefore, it has become impossible to use rings made of cast iron, which have hitherto been generally used, or rings made from carbon steel, silicon-chromium steel or oil-tempered wire. Namely, rings made of cast iron have the drawback that it is difficult to produce rings smaller in size in the axial direction and the breaking strength thereof is unsatisfactory. The silicon-chromium rings are formed relatively large in cross-sectional area, in view of the poor high-temperature strength of silicon-chromium steel, and have a great inertia, which will bring about the fluttering phenomenon. Therefore, tool steels, spring steels and stainless steels have recently come to be used as materials for piston rings. Of stainless steels, particularly, 13Cr martensitic stainless steel (0.65C-13.5Cr-0.3Mo-0.1V) has been used for compression rings to give good results. On the other hand, the oil ring has the important function of appropriately controlling the amount of the lubricating oil at the time of sliding contact between the piston rings and the cylinder and scraping off excess lubricating oil to prevent it from penetrating into the combustion chamber. Therefore, side rails for the oil ring are, like the compression rings, required to have heat resistance and wear resistance. The same material as that for the compression rings has been used for the side rails to give good results.
The piston rings made of the martensitic stainless steel, however, are not satisfactory in wear resistance and scuffing resistance when used for engines in which severe abrasive wear takes place. Compression rings made from martensitic stainless steel and subjected to a gas nitriding treatment are unsatisfactory in strength of fitting to the piston, and has the problem that they may be broken when the joint gap is excessively widened (to 10T or above, effective durability being 11-13T, where T is thickness (mm) of ring). Further, such compression rings have the drawback of being unsatisfactory in scuffing resistance and, therefore, being scuffed when used for internal combustion engines in which scuffing resistance requirements are severe. Accordingly, a thin Ni-P or Ni-Co-P plating or such base plating with hard particles (e.g., Si.sub.3 N.sub.4) dispersed therein has been provided only on a sliding surface of the compression ring. In view of the above, in connection with the piston rings made of martensitic stainless steel there has been a demand for further higher wear resistance and scuffing resistance in order to prolong the service life of the piston rings.
In the internal combustion engines, rocker arms are operated in abutment with a cam shafts. As the cam shafts rotate in high rotational speed, the rocker arms are required to be resistant to wear and scuffing.
Further, other shafts used in automobiles and operated under severe sliding condition with heavy load, such as pinion shafts of pinion gears used in differential gear device for front-engine front-wheel-drive vehicles, are required to be resistant in seizure and to wear.